Emergent nanoscale superparamagnetism at oxide interfaces

Atomically sharp oxide heterostructures exhibit a range of novel physical phenomena that do not occur in the parent bulk compounds. The most prominent example is the appearance of highly conducting and superconducting states at the interface between the band insulators LaAlO3 and SrTiO3. Here we report a new emergent phenomenon at the LaMnO3/SrTiO3 interface in which an antiferromagnetic insulator abruptly transforms into a magnetic state that exhibits unexpected nanoscale superparamagnetic dynamics. Upon increasing the thickness of LaMnO3 above five unit cells, our scanning nanoSQUID-on-tip microscopy shows spontaneous formation of isolated magnetic islands of 10 to 50 nm diameter, which display random moment reversals by thermal activation or in response to an in-plane magnetic field. Our charge reconstruction model of the polar LaMnO3/SrTiO3 heterostructure describes the sharp emergence of thermodynamic phase separation leading to nucleation of metallic ferromagnetic islands in an insulating antiferromagnetic matrix. The model further suggests that the nearby superparamagnetic-ferromagnetic transition can be gate tuned, holding potential for applications in magnetic storage and spintronics

Probing dynamics and pinning of single vortices in superconductors at nanometer scales

The dynamics of quantized magnetic vortices and their pinning by materials defects determine electromagnetic properties of superconductors, particularly their ability to carry non-dissipative currents. Despite recent advances in the understanding of the complex physics of vortex matter, the behavior of vortices driven by current through a multi-scale potential of the actual materials defects is still not well understood, mostly due to the scarcity of appropriate experimental tools capable of tracing vortex trajectories on nanometer scales. Using a novel scanning superconducting quantum interference microscope we report here an investigation of controlled dynamics of vortices in lead films with sub-Angstrom spatial resolution and unprecedented sensitivity. We measured, for the first time, the fundamental dependence of the elementary pinning force of multiple defects on the vortex displacement, revealing a far more complex behavior than has previously been recognized, including striking spring softening and broken-spring depinning, as well as spontaneous hysteretic switching between cellular vortex trajectories. Our results indicate the importance of thermal fluctuations even at 4.2 K and of the vital role of ripples in the pinning potential, giving new insights into the mechanisms of magnetic relaxation and electromagnetic response of superconductors.Comment: 15 pages and 5 figures (main text) + 15 pages and 11 figures (supplementary material

Thermoelectric spin voltage in graphene

In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents. Amongst the most intriguing phenomena is the spin Seebeck effect, in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect. Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport, energy-dependent carrier mobility and unique density of states. Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current. These results could prove crucial to drive graphene spintronic devices and, in particular, to sustain pure spin signals with thermal gradients and to tune the remote spin accumulation by varying the spin-injection bias

Determination of the spin-lifetime anisotropy in graphene using oblique spin precession

We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low-and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields

Quantification of the flux tubes and the stability of stripe pattern in the intermediate state of a type-1 superconducting film

The intermediate state in a type-1 superconducting Pb film is studied by using the scanning Hall probe microscopy, which shows quantized flux tubes with distinct flux density. The vorticity of flux tubes are quantified using the monopole model. It is found that the vorticity of the flux tubes can be tuned by using flux expulsion process under different magnetic field and temperatures. The stability of stipe patterns at high fields is studied with new stripe patterns formed after shaking with ah ac field. No flux tube is observed even after shaking with intense ac fields. All the results suggests the stripe patterns have very close energy, which is much favorable than the flux tube state. © 2014 Elsevier B.V. All rights reserved.status: publishe

Observation of single flux quantum vortices in the intermediate state of a type-I superconducting film

The flux quantization in the intermediate state of a type-I superconducting Pb film is studied by using scanning Hall probe microscopy. The vorticity of flux tubes can be tuned by changing the cooling field through the flux expulsion process, and single flux quantum vortices coexisting with multiple quantized flux tubes are observed at low enough fields. However, the minimum fluxoid observed through flux penetration is found to contain more than one flux quantum, and its vorticity increases with decreasing temperature. By combining these two processes it is possible to stabilize flux tubes of opposite polarity, and single flux quantum vortices are created through the annihilation process under the drive of the Lorentz force. Our results give strong evidence that single quantum vortices can be thermodynamically stabilized in the intermediate state of type-I superconductors. © 2013 American Physical Society.status: publishe

Nanoscale thermal imaging of dissipation in quantum systems

arXiv:1609.01487.-- et al.Energy dissipation is a fundamental process governing the dynamics of physical, chemical and biological systems. It is also one of the main characteristics that distinguish quantum from classical phenomena. In particular, in condensed matter physics, scattering mechanisms, loss of quantum information or breakdown of topological protection are deeply rooted in the intricate details of how and where the dissipation occurs. Yet the microscopic behaviour of a system is usually not formulated in terms of dissipation because energy dissipation is not a readily measurable quantity on the micrometre scale. Although nanoscale thermometry has gained much recent interest, existing thermal imaging methods are not sensitive enough for the study of quantum systems and are also unsuitable for the low-temperature operation that is required. Here we report a nano-thermometer based on a superconducting quantum interference device with a diameter of less than 50 nanometres that resides at the apex of a sharp pipette: it provides scanning cryogenic thermal sensing that is four orders of magnitude more sensitive than previous devices-below 1 μKH . This non-contact, non-invasive thermometry allows thermal imaging of very low intensity, nanoscale energy dissipation down to the fundamental Landauer limit of 40 femtowatts for continuous readout of a single qubit at one gigahertz at 4.2 kelvin. These advances enable the observation of changes in dissipation due to single-electron charging of individual quantum dots in carbon nanotubes. They also reveal a dissipation mechanism attributable to resonant localized states in graphene encapsulated within hexagonal boron nitride, opening the door to direct thermal imaging of nanoscale dissipation processes in quantum matter.This work was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 programme (grant no. 655416), by the Minerva Foundation with funding from the Federal German Ministry of Education and Research, and by a Rosa and Emilio Segré Research Award. L.S.L. and E.Z. acknowledge the support of the MISTI MIT-Israel Seed Fund.Peer Reviewe

Superconductivity in Pb cluster assembled systems with different degrees of coagulation

Superconducting properties are observed and investigated in Pb nanogranular systems prepared by deposition of clusters produced in a laser vaporization cluster source. Different morphologies were achieved by controlling the degree of coagulation via the substrate temperature and the magnetic response of these systems was studied. Deposition on substrates at temperatures above 200 degrees C results in an ensemble of weakly coupled islands showing superconducting confinement effects. Cluster deposition on cooled substrates limits the coagulation and results in cluster assembled thin films with strong intergrain coupling. This method provides a unique way to produce systems with very high flux pinning leading to avalanche effects.status: publishe

Effect of temperature on the growth of single crystalline monolayer graphene by Chemical Vapor Deposition (CVD)

Resumen del póster presentado a la 6th edition of Graphene Conference series, the largest European Event in Graphene and 2D Materials, celebrada en Genova (Italia) del 19 al 22 de abril de 2016.The ever increasing interest in graphene properties and its applications has motivated the controlled growth of high-quality graphene and fabrication of graphene-based devices. The growth of graphene via CVD using metal catalysts depends on both the intrinsic properties of the metal catalysts and the growth parameters. Here we demonstrate that the structure of single layer graphene flakes grown on a copper substrate by low pressure CVD depends dramatically on the furnace temperature, within a few tens of degrees Celsius. Optical microscope analysis of as-grown and transferred graphene onto SiO2/Si shows that growth at 1000ºC results in dendritic shapes while growth at 1040ºC gives a compact graphene flake. The low temperature growth was extended over a long time (1 hour) in order to check if there was a change in the structure towards a compact flake as the one in Figure b, which was obtained after just 10 minutes of growth time at 1040ºC. However, the size of the dendrites increased without merging. Although still poorly understood, the dendritic growth may be due to the poor smoothening of the copper at the lower annealing temperatures and to the low carbon attachment/detachment kinetics at the graphene growth fronts. We have characterized the charge and spin transport properties of the graphene grown at low temperatures. We have fabricated non-local spin valve devices with 3 μm graphene channel length and found a spin life time of 0.2 ns and spin diffusion length of 2.5 μm at room temperature. The mobility of the device was of 1000 cm2 /Vs, which is typical for CVD grown graphene on SiO2/Si. Future work will focus on comparing these results with the spintronic performance of graphene grown at higher temperatures.Peer Reviewe